Pow! a firecracker explodes close to you. your mouth drops open in surprise, a movement known as:

When the mouth opens wide and your chin drops, the movement is known as depression movement, which occurs in the temporomandibular joint. This movement occurs naturally when we are surprised or shocked due to a certain event, it is also considered to be a common facial expression.

Answer is in the photo. I couldn't attach it here, but uploaded it to a file hosting. link below! Good Luck!

cutt.ly/4zZs6GM